Make description in user.addstatus really optional

Summary:
Fixes:

Column 'description' cannot be null".

Test Plan:

$ echo '{"fromEpoch": 1, "toEpoch": 2, "status": "away"}' | arc call-conduit user.addstatus

Reviewers: btrahan, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4189